1. Field of the Invention
This invention relates to an overheat detection system for an outboard motor, more particularly an overheat detection system of an internal combustion engine for an outboard motor for small boats
2. Description of the Related Art
Small motor-driven boats are generally equipped with a propulsion unit including an internal combustion engine, propeller shaft and propeller integrated into what is called an outboard motor or engine. The outboard motor is mounted on the outside of the boat and the output of the engine is transmitted to the propeller through a clutch and the propeller shaft. The boat can be propelled forward or backward by moving the clutch from Neutral to Forward or Reverse position.
The engine is usually provided with an overheat detection system which detects the engine temperature by a sensor and if the detected engine temperature is found to exceed a permissible temperature, it determines that the engine is overheated and warns the operator. For example, Japanese Laid-Open Patent Application No. Hei 11 (1999)-200862 teaches such an overheat detection for an engine mounted on a tractor.
While this prior art system is for an engine mounted on a machine used on land, when the engine is an outboard engine, sea or lake water is used for engine cooling. Specifically, when the outboard engine has been warmed, a thermostat provided in the engine cooling passage is opened to introduce sea walter (or lake water) into the engine to cool the same.
Disadvantageously, the temperature sensor may, depending on its location, indicate a temperature higher than a true or actual engine temperature, due to a response lag in the thermostat or the layout of the cooling passage, which makes it difficult to detect an engine overheated condition accurately. This disadvantageous is serious at high engine load immediately after engine starting.
An object of the present invention is therefore to solve the problem by providing an overheat detection system of an internal combustion engine for an outboard motor, which can detect the overheat condition accurately, even if the temperature sensor, depending on its location, indicates a temperature higher than a true temperature at the time of, for example, high engine load immediately after engine starting.
For realizing this object, there is provided a system for detecting overheat of an internal combustion engine for an outboard motor mounted on a boat, whose output is connected to a propeller such that the boat is propelled forward or reverse, comprising: a temperature sensor, installed at the engine, which outputs a signal indicative of a temperature of the engine; a crank angle sensor, installed at the engine, which outputs a signal indicative of a speed of the engine; time measuring means for measuring a period of time since starting of the engine; time determining means for determining whether the measured time is within a predetermined first period of time; temperature determining means for determining whether the detected engine temperature is equal to or greater than a first predetermined temperature; engine speed determining means for determining whether the detected engine speed exceeds a predetermined speed; threshold value changing means for changing a threshold value to a value set in a temperature-increasing direction, when the measured time is determined to be within the predetermined first period of time, the detected engine temperature is determined to be equal to or greater than the first predetermined temperature and the detected engine speed is determined to exceed the predetermined speed; and overheat determining means for comparing the detected engine temperature with the threshold value and for determining that the engine is overheated when the detected engine temperature is determined to exceed the threshold value.